Method of manufacturing of electrolumineschent devices

ABSTRACT

An electroluminescent device includes a semiconductor layer (4) in the form of a thin dense polymer film comprising at least one conjugated polymer, a first contact layer (5) in contact with a first surface of the semiconductor layer, and a second contact layer (3) in contact with a second surface of the semiconductor layer. The polymer film (4) of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers that on applying an electric field between the first and second contact layers across the semiconductor layer so as to render the second contact layer positive relative to the first contact layer charge carriers are injected into the semiconductor layer and radiation is emitted from the semiconductor layer. The polymer film can be poly (p-phenyenevinylene) [PPV] of formula (I) wherein the phenylene ring may optionally carry one or more substituents each independently selected from alkyl (preferably methyl), alkoxy (preferably methoxy or ethoxy), halogen (preferably chlorine or bromine) or nitro. A method of manufacture of an electroluminescent device includes steps of depositing a thin layer of a precursor polymer onto a substrate and then heating the precursor polymer to a high temperature to form the conjugated polymer.

This is a division of application Ser. No. 07/634,117, filed Dec. 28,1990, now U.S. Pat. No. 5,247,190.

FIELD OF THE INVENTION

The present invention relates to electroluminescent (EL) devices, and inparticular EL devices in which the light-emitting layer is asemiconductor.

BACKGROUND OF THE INVENTION

Electroluminescent (EL) devices are structures which emit light whensubject to an applied electric field. The usual model for the physicalprocess in a semiconductor used in this way is through the radiativecombination of electron-hole pairs which are injected into thesemiconductor from opposite electrodes. Common examples arelight-emitting diodes based on GaP and similar III-V semiconductors.Though these devices are efficient and widely used, they are limited insize, and are not easily or economically used in large area displays.Alternative materials which can be prepared over large areas are known,and among the inorganic semiconductors most effort has been directed toZnS. This system has considerable practical drawbacks, primarily poorreliability. The mechanism in ZnS is believed to be one whereacceleration of one type of carrier through the semiconductor under astrong electric field causes local excitation of the semiconductor whichrelaxes through radiative emission.

Among organic materials, simple aromatic molecules such as anthracene,perylene and coronene are known to show electroluminescence. Thepractical difficulty with these materials is, as with ZnS, their poorreliability, together with difficulties in deposition of the organiclayers and the current-injecting electrode layers. Techniques such assublimation of the organic material suffer from the disadvantage thatthe resultant layer is soft, prone to recrystallisation, and unable tosupport high temperature deposition of top-contact layers. Techniquessuch as Langmuir-Blodgett film deposition of suitably-modified aromaticssuffer from poor film quality, dilution of the active material, and highcost of fabrication.

An electroluminescent device utilising anthracene is disclosed in U.S.Pat. No. 3,621,321. This device suffers from high power consumption andlow luminescence. In an attempt to provide an improved device, U.S. Pat.No. 4,672,265 describes an EL device having a double layer structure asits luminescent layer. However, the suggested materials for the doublelayer structure are organic materials which suffer from thedisadvantages mentioned above.

SUMMARY OF THE INVENTION

The present invention seeks to provide an electroluminescent device inwhich the above mentioned drawbacks are obviated or at least mitigated.

The present invention provides in one aspect an electroluminescentdevice comprising a semiconductor layer in the form of a thin densepolymer film comprising at least one conjugated polymer, a first contactlayer in contact with a first surface of the semiconductor layer, and asecond contact layer in contact with a second surface of thesemiconductor layer, wherein the polymer film of the semiconductor layerhas a sufficiently low concentration of extrinsic charge carriers thaton applying an electric field between the first and second contactlayers across the semiconductor layer so as to render the second contactlayer positive relative to the first contact layer charge carriers areinjected into the semiconductor layer and radiation is emitted from thesemiconductor layer.

The invention is based on the discovery by the present inventors thatsemiconductive conjugated polymers can be caused to exhibitelectroluminescence by the injection of charge carriers from suitablecontact layers.

Semiconductive conjugated polymers per se are known. For example, theiruse in an optical modulator is discussed in EP-A-0294061. In that case,polyacetylene is used as the active layer in a modulating structurebetween first and second electrodes. It is necessary to place aninsulating layer between one of the electrodes and the active layer soas to create a space charge region in the active layer which gives riseto the optical modulation effect. Such a structure could not exhibitelectroluminescence since the presence of the space charge layerprecludes the formation of electron/hole pairs whose-decay gives rise toradiation. It will be clear in any event that the exhibition ofelectroluminescence in EP-A-0294061 would be wholly undesirable sincethe optical modulation effect would be disrupted thereby.

In the device of the present invention, the conjugated : polymer ispreferably poly (p-phenylenevinylene) [PPV] and the first chargeinjecting contact layer is a thin layer of aluminium one surface havingformed a thin oxide layer, the first surface of the semiconducting layerbeing in contact with the said oxide layer and the second chargeinjecting contact layer is a thin layer of aluminium or gold.

In another embodiment, the conjugated polymer is PPV, the first contactlayer is aluminium or an alloy of magnesium and silver and the secondcontact layer is indium oxide.

In yet another embodiment, the conjugated polymer is PPV and one of thecontact layers is non-crystalline silicon and the other of the contactlayers is selected from the group consisting of aluminium, gold,magnesium/silver alloy and indium oxide.

These embodiments can be made by putting down either the first contactlayer or the second contact layer onto a substrate, applying a thin filmof PPV and then putting down the other of the first and second contactlayers.

Preferably the polymer film is of substantially uniform thickness in therange 10 nm to 5 μm and the conjugated polymer has a semiconductor bandgap in the range 1 eV to 3.5 ev. Furthermore the proportion of theconjugated polymer in the electroluminescent areas of the polymer filmis sufficient to achieve the percolation threshold for charge transportin the conjugated polymer present in the film.

A second aspect of the present invention provides a method ofmanufacture of an electroluminescent device wherein a semiconductorlayer in the form of a thin layer of a dense polymer film comprising atleast one conjugated polymer is deposited onto a substrate by the stepsof depositing a thin layer of a precursor polymer as a thin polymer filmonto the substrate and then heating the deposited precursor polymer to ahigh temperature to form the conjugated polymer, a thin layer of a firstcontact layer being placed in contact with a first surface of thesemiconductor layer and a thin layer of a second contact layer beingplaced in contact with a second surface of the semiconductor layer,wherein the polymer film has a sufficiently:low concentration ofextrinsic charge carriers that on applying an electric field between thefirst and second contact layers once in contact with the semiconductorlayer so as to render the second contact layer positive relative to thefirst contact layer charge carriers are injected into the semiconductorlayer and radiation emitted from the semiconductor layer.

By conjugated polymer is meant, a polymer which possesses a delocalisedπ-electron system along the polymer backbone; the delocalised π-electronsystem confers semiconducting properties to the polymer and gives it theability to support positive and negative charge carriers with highmobilities along the polymer chain. Such polymers are discussed forexample by R. H. Friend in Journal of Molecular Electronics 4 (1988)January-March, No. 1, pages 37 to 46.

It is believed that the mechanism underlying the present invention issuch that the positive contact layer injects positive charge carriersinto the polymer film and the negative contact layer injects negativecharge carriers into the polymer film, these charge carriers combiningto form charge carrier pairs which decay radiatively. To achieve this,preferably the positive contact layer is selected to have a high workfunction and the negative contact layer to have a low work function.Hence the negative contact layer comprises an electron-injectingmaterial, for example a metal or a doped semiconductor that, when placedin contact with the polymer semiconductor layer and made negative withrespect to the polymer semiconductor through application of an externalpotential across the circuit, allows the injection of electrons into thepolymer semiconductor layer. The positive contact layer comprises ahole-injecting material, for example a metal or a doped semiconductorthat, when placed in contact with the polymer semiconductor layer andmade positive with respect to the polymer semiconductor throughapplication of an external potential across the circuit, allows theinjection of positive charges, commonly termed "holes", into the polymersemiconductor layer.

In order to produce the desired electroluminescence, the polymer filmmust be substantially free of defects which act as non-radiativerecombination centres, since such defects prevent electroluminescence.

By "dense" polymer film is meant that the polymer film is not fibrillarand is substantially free of voids.

One or each of the contact layers can include, in addition to the layerof charge injecting material, a further layer of a material, preferablyan organic material, which serves to control the injection ratio ofelectrons and holes into the EL layer and to ensure that radiative decaytakes place away from the charge injecting material of the contactlayers.

The film of conjugated polymer preferably comprises a single conjugatedpolymer or a single co-polymer which contains segments of conjugatedpolymer. Alternatively, the film of conjugated polymer may consist of ablend of a conjugated polymer or copolymer with another suitablepolymer.

Further preferred features of the polymer film are that:

(i) the polymer should be stable to oxygen, moisture, and to exposure toelevated temperatures;

(ii) the polymer film should have good adhesion to an underlying layer,good resistance to thermally-induced and stress-induced cracking, andgood resistance to shrinkage, swelling, recrystallisation or othermorphological changes;

(iii) the polymer film should be resilient to ion/atomic migrationprocesses, e.g. by virtue of a high crystallinity and high meltingtemperature.

Embodiments of the present invention will now be described by way ofexample only, with reference to the accompanying drawings.

The film of conjugated polymer is preferably a film of apoly(p-phenylenevinylene) [PPV] of formula ##STR1## wherein thephenylene ring may optionally carry one or more substituents eachindependently selected from alkyl (preferably methyl), alkoxy(preferably methoxy or ethoxy), halogen (preferably chlorine orbromine), or nitro.

Other conjugated polymers derived from poly(p-phenylenevinylene) arealso suitable for use as the polymer film in the EL devices of thepresent invention. Typical examples of such derivatives are polymersderived by:

(i) replacing the phenylene ring in formula (I) with a fused ringsystem, e.g. replacing the phenylene ring with an anthracene ornaphthalene ring system to give structures such as: ##STR2##

These alternative ring systems may also carry one or more substituentsof the type described above in relation to the phenylene ring.

(ii) replacing the phenylene ring with a heterocyclic ring system suchas a furan ring to give structures such as: ##STR3##

As before, the furan ring may carry one or more substituents of the typedescribed above in relation to phenylene rings.

(iii) increasing the number of vinylene moieties associated with eachphenylene ring (or each of the other alternative ring systems describedabove in (i) and (ii)) to give structures such as: ##STR4## wherein yrepresents 2, 3, 4, 5, 6, 7 . . .

Once again, the ring systems may carry the various substituentsdescribed above.

These various different PPV derivatives will possess differentsemiconductor energy gaps; this should permit the construction ofelectroluminescent devices which have emission at different wavelengthscovering the entire visible part of the spectrum.

The film of conjugated polymer may be prepared by means of a chemicaland/or thermal treatment of a solution--processible or melt-processible"precursor" polymer. The latter can be purified or pre-processed intothe desired form before subsequent transformation to the conjugatedpolymer via an elimination reaction.

Films of the various derivatives of PPV described above can be appliedonto a conducting substrate in similar manner by using an appropriatesulphonium precursor.

In certain circumstances it may be advantageous to use polymerprecursors which have a higher solubility in organic solvents than thesulphonium salt precursors (II). Enhanced solubility in organic solventscan be achieved by replacing the sulphonium moiety in the precursor by aless hydrophilic group such as an alkoxy group (usually methoxy), or apyridinium group.

Typically, a film of poly(phenylinevinylene) is applied onto aconducting substrate by a method which relies on a reaction scheme suchas is illustrated in FIG. 1. The sulphonium salt monomer (II) isconverted into a precursor polymer (III) in aqueous solution or in asolution of methanol/water, or methanol. Such a solution of thepre-polymer (III) can be applied onto a conducting substrate by means ofstandard spin-coating techniques as used in the semiconductor industryfor photoresist processing. The resultant film of precursor-polymer IIIcan then be converted into poly(phenylene vinylene) (I) by heating totemperatures typically in the range 200°-350° C.

Details of the conditions necessary for the chemical synthesis of themonomer (II), its polymerisation to the precursor (III) and its thermalconversion to PPV are described in the literature, for example in D. D.C. Bradley, J. Phys. D (Applied Physics) 20, 1389 (1987); and J. D.Stenger Smith, R. W. Lenz and G. Wegner, Polymer 30, 1048 (1989).

We have found that with poly(phenylenevinylene) films of a thickness inthe range 10 nm to 10 micrometres can be obtained. These PPV films arefound to have very few pin holes. The PPV film has a semiconductorenergy gap of about 2.5 eV (500 nm); it is robust, shows little reactionwith oxygen at room temperature, and is stable out of air attemperatures well in excess of 300° C.

Enhanced ordering in the conjugated material may be achieved bymodifying the leaving group of the precursor polymer to ensure that theelimination proceeds smoothly via a simple reaction without generationof additional intermediate structures. Thus, for example, the normaldialkyl sulphonium moiety can be replaced with a tetrahydrothiopheniummoiety. The latter eliminates as a single leaving group withoutdecompositon, as is seen for dialkyl sulphide, into an alkyl mercaptan.In the examples described here, the precursor polymers used include boththat with the dialkyl sulphonium moiety chosen as dimethyl sulphide andas tetratryebrothiophene. Both precursors produce film of PPV suitablefor use in the device structures shown in examples below.

A further material which may be suitable for forming the film ofconjugated polymer is poly(phenylene).

This material may be prepared by starting from biologically synthesisedderivatives of 5,6-dihydroxycyclohexa-1,3-dienes. These derivatives canbe polymerised by use of radical initators into a polymer precursor thatis soluble in simple organic solvents. This preparation ofpoly(phenylene) is more fully described in Ballard et al, J. Chem. Soc.Chem. Comm. 954 (1983).

A solution of the polymer precursor can be spin coated as a thin filmonto a conducting substrate and then be converted to the conjugatedpoly(phenylene) polymer by a heat treatment, typically in the range 140°to 240° C.

Copolymerisation with vinyl or diene monomers can also be performed soas to obtain phenylene copolymers.

A further category of materials which can be used to form the requiredfilm of conjugated polymer is a conjugated polymer which is itselfeither solution processible or melt processible by virtue of thepresence of bulky pendant side groups attached to the main conjugatedchain or by virtue of the inclusion of the conjugated polymer into acopolymer structure of which one or more components are non-conjugated.Examples of the former include:

(a) Poly(4,4'-diphenylenediphenylvinylene) [PDPV] is an arylene vinylenepolymer in which both vinylene carbons are substituted by phenyl rings.It is soluble in common organic solvents thus enabling the preparationof thin films.

(b) Poly(1,4-phenylene-1-phenylvinylene) andpoly(1,4-phenylenediphenylvinylene) polymers are analogues of PPV inwhich respectively one and both vinylene carbons are substituted withphenyl groups. They are both soluble in organic solvents and may be castor spun into thin film form.

(c) Poly(3-alkylthiophene) polymers (alkyl is one of propyl, butyl,pentyl, hexyl, heptyl, octyl, decyl, undecyl, dodecyl etc) which aresolution processible in common organic solvents and which for longeralkyl sequences (alkyl greater than or equal to octyl) are also meltprocessible.

(d) Poly(3-alkylpyrrole) polymers which are expected to be similar tothe poly(3-alkylthiophene) polymers.

(e) Poly(2,5-dialkoxy-p-phenylenevinylene) polymers with alkyl greaterthan butyl are solution processible.

(f) Poly(phenylacetylene) is a derivative of polyacetylene in which thehydrogen atoms along the chain are replaced by phenyl groups. Thissubstitution renders the material soluble.

In some circumstances it may also be appropriate to form polymer blendsof the conjugated polymer with other polymers so as to obtain therequired processibility of the polymer and thereby facilitate forming ofthe required thin uniform films of the polymer on the conductingsubstrate (the charge injecting contact layer).

When such copolymers or polymer blends are used to form the film ofconjugated polymer, the active region of the electroluminescent devicewhich incorporates the said film of conjugated polymer must contain avolume fraction of conjugated polymer which is greater than or equal tothe percolation threshold of the copolymer or polymer blend.

The semiconductor electroluminescent layer may be formed as a compositelayer with layers of polymers with different band gaps and/or majoritycharge species so that, for example, concentration of the injectedcharge, from the charge injecting contact layer, within a particularregion of the EL device may be achieved. Composite layers may befabricated by successive deposition of polymer layers. In the case wherefilms are deposited in the form of the precursor by spin- ordraw-coating to the conjugated polymer, the conversion process to theconjugated polymer renders the film insoluble, so that subsequent layersmay be similarly applied without dissolution of the previously depositedfilm.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how thesame may be carried into effect, reference will now be made, by way ofexample to the accompanying drawings in which:

FIG. 1 is a formulae drawing showing a reaction scheme for laying downthe conjugated polymer;

FIGS. 2 and 3 are sketches of an electroluminescent device in accordancewith the present invention;

FIG. 4 is a graph of the electroluminescent output of the devicedescribed with reference to FIGS. 2 and 3;

FIGS. 5 and 6 are graphs of current flow VS light emission, and outputintensity VS applied voltage, respectively for an electroluminescentdevice according to another example of the invention; and

FIGS. 7 and 8 are graphs respectively of the current output andelectroluminescent intensity of a further example of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

Referring now to FIGS. 2 and 3 an EL device was constructed as follows:

Onto the upper surface of a substrate of glass for example a silica orborosilicate glass 1 of approximately 1 mm thickness, a first chargeinjecting contact layer 2 was formed. The charge injecting contact layerwas formed by thermal evaporation of aluminium through a shadow-maskresulting in a layer of approximately 20 nm in thickness. Theshadow-mask was used to define a pattern which was a series of parallelstrips of width 2 mm, separation 2 mm and length 15 mm. The resultingaluminium charge injecting contact layer was then exposed to the air toallow formation of a thin surface oxide layer 3. This then formed theelectron injecting contact layer.

A solution of the precursor to PPV, in methanol, having a concentrationin the range 1 gram polymer to 10 to 25 ml of methanol, was spin-coatedonto the combination substrate described above. This was achieved byspreading the polymer solution over the whole surface of the combinationsubstrate and then spinning the substrate, held with its upper surfacehorizontal, about a vertical axis at speeds of up to 5000 r.p.m. Theresultant substrate and precursor polymer layer was then heated in avacuum oven at a temperature of 300° C. for 12 hours. This heattreatment converted the precursor polymer to PPV, and the resultant PPVfilm 4 had a thickness in the range 100 to 300 nm. The minimumrequirements for the film thickness is set by the film conductance and alower limit is in the region of 20 nm. However, the preferred range ofthickness is 20 nm to 1 μm.

A second charge injecting contact layer (5) was then formed by theevaporation of gold or aluminium onto the PPV film. A shadow mask wasagain used to define a pattern on the surface of the PPV film so that aseries of parallel strips of width 2 mm, separation 2 mm and length 15mm was formed rotated at right angles to the first charge injectingcontact layer strips. The thickness of the second charge injectingcontact layer was in the range 20-30 nm. This then formed thehole-injecting contact layer.

It, is preferable that at least one of the charge injecting contactlayers is transparent or semitransparent in order to allow lightemission from the EL device perpendicular to the plane of the device.This is achieved here with aluminium and gold layers of a thickness notexceeding 30 mm. For a device with the thickness of the PPV layer about200 nm, the threshold voltage for charge injection and the appearance ofstrong electroluminescence is about 40 volts. The voltage gives athreshold electric field of 2×10⁶ Vcm⁻¹. At a current density of 2mA/cm² the light emission through the semitransparent electrodes wasvisible by eye under normal lighting conditions. The output of thedevice showed only weak dependance on frequencies up to 100 kHz. Thisdemonstrates that the response time of the EL device is very short andis faster than 10 microseconds. When in use the EL device was operatedin air with no special precautions taken and exhibited no obviousindications of degradation.

The light output from the device was spectrally resolved with a gratingmonochromator and detected with a silicon photovoltaic cell, andmeasurements were performed both at room temperature (20° C.) and alsowith the device held in a cryostat with optical access, at lowtemperatures. Results are shown in FIG. 4. The EL spectrum shows lightoutput over the spectral range 690 to 500 nm (1.8 to 2.4 eV) with peaksseparated by about 0.15 eV which shift in position a little withtemperature.

Other materials which are suitable for use as an electron-injectingcontact layer because they have a low work function relative to the ELlayer are: n-doped silicon (amorphous or crystalline), silicon with anoxide coating, alkali and alkaline-earth metals either pure or alloyedwith other metals such as Ag. Also thin layers of "n-type doped"conjugated polymers may be interposed between a metallic layer and theelectroluminescent polymer layer to form the electron-injecting contactlayer.

Other materials which are suitable for use as a hole-injecting contactlayer because they have a high work function relative to the EL layerare: indium/tin oxides (which are transparent in, the visible part ofthe spectrum), platinum, nickel, palladium and graphite. Also thinlayers of "p-type doped" conjugated polymers, such as electrochemicallypolymerised polypyrrole or polythiophene may be interposed between ametallic layer and the electroluminescent polymer layer to form thehole-injecting contact layer.

The above mentioned materials may be applied as follows: all metalsexcept those with very high melting point temperatures such as platinummay be deposited by evaporation; all metals including indium/tin oxidemay be deposited using DC or RF sputtering and also electron beamevaporation; for amorphous silicon deposition may be done byglow-discharge deposition from mixtures of silane and dopants such asphosphine.

The following are some examples of structures using these materials.

Example 2

The structure for this example is built up as a series of layers on aglass substrate. First, a layer of conducting but transparent indiumoxide was deposited onto the substrate by a process involving ion-beamsputtering from an indium target in the presence of oxygen.

Samples are prepared in cryopumped system with a base pressure of 10⁻⁸mbar. The substrate is water-cooled, remaining at room temperature forall depositions used here. Ion-beam sputtering from an indium target atdeposition rates of typically 0.1 nm/sec, in the presence of an oxygenpressure of typically 2×10⁻⁴ mbar, produced films of transparent indiumoxide with resistivities of typically 5×10⁻⁴ Ω cm. Typical thicknessesof 100 nm give specific sheet resistances of about 50 Ω per square. Suchfilms have optical transmission coefficients in the visible portion ofthe spectrum of better than 90%.

These films have an amorphous structure, as determined from X-ray andelectron diffraction measurements.

A layer of PPV is next deposited onto the indium oxide layer, using theprocedure described in example 1 above. A top contact of aluminium isfinally deposited by evaporation, typically to a thickness of 50 nm.This structure is operated with the indium oxide contact layerfunctioning as the positive contact, and the aluminium contact as thenegative contact. Light emission is viewed through the indium oxidelayer.

Results for a structure constructed this way, with a layer of PPV ofthickness 70 nm, and an active area of 2 mm² are shown in FIGS. 5 and 6.The threshold for current flow associated with light emission is seen tobe at about 14V in FIG. 5. The variation in the intensity of thespectrally-integrated light output for the device is shown as a functionof current in FIG. 6.

Example 3

The fabrication of this structure is as for Example 2 above up to thetop metal contact. Here, we deposit by evaporation an alloy of silverand magnesium to form the top contact which acts as the negativecontact. The evaporation is performed by heating a mixture of silver andmagnesium powders in a molar ration of 1 to 10 in a boat, and filmthicknesses of typically 50 nm were deposited.

Magnesium is desirable as a material for the negative electrode as ithas a low work function. The addition of silver to form an alloyimproves the adhesion of the metal film to the polymerlayer, andimproves its resistance to oxidation. The current/voltage and ELproperties of these samples were similar to those described in Example2.

Example 4

These structures were fabricated with a layer of amorphoussilicon-hydrogen alloy acting as the negative electrode, and indiumoxide as the positive electrode. A glass substrate is used with anevaporated Metal contact layer of aluminium or chromium. The amorphoussilicon-hydrogen film was then deposited by radio-frequency, RF,sputtering as detailed below.

The RF sputterer used has two targets, a liquid Nitrogen cooled getterand is operated with a target-substrate separation of 8 cm. The chamberhas a base pressure of 5×10⁻⁸ mbar. Magnetron targets are loaded withlayers of n-doped Si wafers to a thickness of 3 mm. The targets arecleaned by presputtering for 1-2 hours prior to sample deposition.,Substrates prepared as above are radiatively heated so that thetemperature at the back side of the 3 cm thick Cu and Al substrate plateis at 250°-300 ° C.

Substrates are rotated at about 6 revs/min. The sputtering gas used is30% H₂ in Ar, at a pressure of 0.007-0.013 mbar, and is continuallypassed through the chamber during deposition. The RF power used is 250Wwith a reflected power of 2W. Deposition rates are typically 12 nm/mingiving deposition times of 1.5 hours for film thicknesses of 1 μm.

The resulting amorphous Si is reddish brown in colour and has a d.c.resistivity of between 5×10⁶ and 5×10⁸ Ω cm. [This found by evaporatingtwo Al pads, either above or below the sample, of length 3 mm and with aseparation of 0.25 mm and measuring the resistance between these twocontacts].

A layer of PPV was then applied to the amorphous silicon-hydrogen layer,as described in Example 1 above, and this was followed with a layer ofindium oxide, deposited directly onto the PPV layer, using the proceduredescribed in Example 2.

Results obtained for a structure fabricated using the steps outlinedabove are shown in FIGS. 7 and 8 for a structure of area 14 mm², andlayer thicknesses of 1 μm for the silicon-hydrogen, 40 nm for the PPVand 250 nm for the indium oxide. FIG. 7 shows the current versus voltagecharacteristic for the device in forward bias (indium oxide positive),and FIG. 8 shows the variation in integrated light output with current.The onset of charge injection and light emission is at about 17V, andthe rise in current above this threshold is, due to the presence here ofthe resistive silicon-hydrogen layer, more gradual than observed instructures without it, as seen for example in FIG. 5.

Structures of this type did also show weaker EL in reverse bias (indiumoxide contact negative with respect to the silicon-hydrogen contact).The preferred mode of operation, however, is in forward bias.

Example 5

Fabrication as in example 4, but with the top layer of indium oxidereplaced by a layer of semitransparent gold or aluminium. Structuresfabricated with the top layer of thickness about 20 nm showed EL throughthis top contact. These devices showed similar characteristics to theexamples discussed above.

The method of fabrication of Example 4 could also be used with thecontact layers described in Examples 2 and 3.

There are other methods, known Der se, for depositing silicon/hydrogen.layers and indium oxide layers. For silicon this could include glowdischarge of silane and evaporation. For indium oxide otherpossibilities-include tin with the indium, to form indium tin oxide(ITO), which has very similar electrical properties to the indium oxidethat we have used here. Deposition methods include evaporation, RF andDC sputtering.

The choice of thickness for the charge injecting contact layers will bedetermined by the deposition technique used and also the desired opticaltransparency of the contact layer. The ease of charge injection may beimproved by constructing the charge injecting contact layers ascomposites. Such composites would incorporate thin layers of oxidisedand reduced conjugated polymers for hole and electron injectionrespectively. These extra layers of conjugated polymer may or may not bethe same as the active electroluminescent polymer layer. Methods ofdoping such materials are well known in the field and are clearlydescribed in "Handbook of Conducting Polymers" T. J. Skotheim.

Although in certain circumstances it-is preferable that at least one ofthe charge injecting contact layers is transparent or semitransparent inorder to allow emission of radiation perpendicular to the plane of thedevice it is not necessarily the case for example when emission withinthe plane of the device only is required.

The limit to the size of the EL device produced is determined-by thesize of the substrate which can be used for spin-coating. For example,15 cm diameter silicon wafers have been coated in this way. To coat muchlarger areas, techniques such as draw-coating may be used instead. It istherefore feasible that EL devices using conjugated polymers with areasof square meters may be constructed.

At least some of the conjugated polymers, including PPV, are capable ofwithstanding post-processing such as the deposition of metal layers inwhich the deposition is required to be at very high temperatures forevaporation, or the deposition of amorphous silicon layers, followed byphotolithographic processes for the definition of activeelectroluminescent areas. Although it is preferable, with the use ofPPV, for either spin- or draw-coating to be used as the methods forapplying the precursor polymer to the substrate depending upon theconjugated polymer and the type of EL device required spin-,draw-coating and melt-processing are all methods which may be used todeposit the conjugated polymer onto the substrate.

The EL device may be used in a variety of ways where electroluminescenceis of use. It may be used where semiconductor LED's have traditionallybeen used. It may also be used where traditionally liquid crystals havebeen used, the EL device having many properties which make it adesirable alternative to liquid crystals.

Since the EL device is light-emitting in contrast to liquid crystaldisplays, the viewing angle is wide. Furthermore, large area EL devicescan be achieved where problems associated with substrate flatness andspacing have been encountered with large area liquid crystal displays.The EL devices are particularly suitable for matrix-addressed displaysfor example televisions and computer displays. An example ofelectroluminescent devices for Use in a matrix-addressed displays isshown in FIG. 3 where the charge injecting contact layers are applied instrips, either side of the semiconductor layer, the strips of onecontact layer being orthogonal to the strips of the other contact layer.The matrix-addressing of individual EL devices or areas of thesemiconductor layer called pixels of the display is achieved by theselection of a particular strip in the lower charge injecting contactlayer and a particular strip, at right angles to the first strip, in theupper charge injecting contact layer. Furthermore since the EL devicehas such a high speed of response then the EL device is suitable for useas a television screen, particularly since the colour of the emittedlight may be controlled through the choice of the conjugated polymer andso its semiconductor band gap and so colour displays using green, redand blue pixels, suitable for colour mixing, are possible through thelocation of different conjugated polymers in the EL device.

Industrial Application

EL devices may also be used as individual shaped elements for indicatorsin vehicle dashboards, on cookers or video recorders for example. Eachelement may be produced in the required shape for the intendedapplication. Furthermore the EL device need not be flat and could, forexample, be formed after fabrication, to follow contours in threedimensions for example the contours of a windscreen in a vehicle oraircraft. For such use the precursor polymer would have to be applied toa suitable substrate such as transparent polymer film such as polyester,polyvinylidene fluoride or polyimide. If the precursor polymer isapplied to such flexible polymer films then continuous fabrication ofthe EL device, onto a roll, is possible. Alternatively the precursorpolymer may be applied, using for example a draw-coating process, onto apre-fabricated contoured substrate.

Finally use of the EL devices is envisaged in optical communicationswhere the EL device may be fabricated directly onto a prepared structureto act as a light source with efficient optical coupling of the ELdevice with optical fibres and/or thin-film waveguides. A similarapplication is described in an article by Satoshi Ishihara in Scienceand Technology in Japan of July 1989, pages 8 to 14 entitled "OpticalInformation Processing".

EL device light sources may be suitable for use as lasers.

We claim:
 1. A method of manufacture of an electroluminescent devicewherein a semiconductor layer in the form of a thin layer of a densepolymer film comprising at least one conjugated polymer is depositedonto a substrate by the steps of depositing a thin layer of a precursorpolymer as a thin polymer film onto the substrate and then heating thedeposited precursor polymer to a high temperature sufficient to convertthe precursor polymer to the conjugated polymer, a thin layer of a firstcontact layer being placed in contact with a first surface of thesemiconductor layer and a thin layer of a second contact layer beingplaced in contact with a second surface of the semiconductor layer,wherein the polymer film has a sufficiently low concentration ofextrinsic charge carriers that on applying an electric field between thefirst and second contact layers once in contact with the semiconductorlayer so as to render the second contact layer positive relative to thefirst contact layer charge carriers are injected into the semiconductorlayer and radiation emitted from the semiconductor layer.
 2. A method asclaimed in claim 1, comprising the steps of firstly depositing the firstcharge injecting contact layer onto a supporting substrate to form acomposite substrate then depositing the precursor polymer as a thindense polymer film onto the first charge injecting contact layer andthen heating the composite substrate and the precursor polymer to a hightemperature to form the conjugated polymer in the polymer film andfinally depositing the second charge injecting contact layer onto thepolymer film.
 3. A method as claimed in claim 1, wherein the precursorpolymer is soluble and is deposited as a thin polymer film on thesubstrate by spin-coating.
 4. A method as claimed in claim 1, whereinthe precursor polymer is a precursor polymer for poly(p-phenylenevinylene) [PPV].
 5. A method as claimed in claim 4, whereinthe thin dense polymer film is of uniform thickness in the range 100 nmto 5 μm.
 6. A method as claimed in claim 4, wherein the first chargeinjecting contact layer is a thin layer of aluminum on one surface ofwhich there is a thin oxide layer, the thin oxide layer of the firstcharge injecting contact layer being placed in contact with the firstsurface of the semiconductor layer.
 7. A method as claimed in claim 4,wherein the second charge injecting contact layer is selected from thegroup consisting of aluminum and gold.
 8. A method as claimed in claim4, wherein the first contact layer is selected from the group consistingof aluminum and magnesium/silver alloy and the second contact layer isindium oxide.
 9. A method as claimed in claim 4, wherein the firstcontact layer comprises amorphous silicon and the second contact layeris selected from the group consisting of aluminum, gold and indiumoxide.
 10. A method as claimed in claim 4, wherein the first and secondcharge injecting contact layers are deposited by evaporation.
 11. Amethod as claimed in claim 4, wherein the supporting substrate is silicaglass.
 12. A method as claimed in claim 4, wherein the substrate isflexible.
 13. A method as claimed in claim 4, wherein the substrate isconstructed of a flexible polymer film.
 14. A method as claimed in claim1, wherein the thin dense polymer film is of uniform thickness in therange 10 nm to 5 μm.
 15. A method as claimed in claim 1, wherein thefirst charge injecting contact layer is a thin layer of aluminum on onesurface of which there is a thin oxide layer, the thin oxide layer ofthe first charge injecting contact layer being placed in contact withthe first surface of the semiconductor layer.
 16. A method as claimed inclaim 1, wherein the second charge injecting contact layer is selectedfrom the group consisting of aluminum and gold.
 17. A method as claimedin claim 1, wherein the first contact layer is selected from the groupconsisting of aluminum and magnesium/silver alloy and the second contactlayer is indium oxide.
 18. A method as claimed in claim 1, wherein thefirst contact layer comprises amorphous silicon and the second contactlayer is selected from the group consisting of aluminum, gold and indiumoxide.
 19. A method as claimed in claim 1, whereinthe first and secondcharge injecting contact layers are deposited by evaporation.
 20. Amethod as claimed in claim 1, wherein the supporting substrate is silicaglass.
 21. A method as claimed in claim 1, wherein the substrate isflexible.
 22. A method as claimed in claim 1, wherein the substrate isconstructed of a flexible polymer film.